Radiation characteristics

X-ray spectroscopy Analytical method by which a sample is irradiated with X-rays, characteristic radiation being emitted after scattering from the specimen. The detection limits for various elements are of the ordering cm. ... 

Jin x-ray monochromator. A monochromator is a large single crystal (usually graphite) that is oriented so that a very iatense reflection is directed toward the sample. AH wavelengths are absorbed by the monochromator except a small range of wavelengths used for the diffraction experiment. Usually only the characteristic radiation is used if an x-ray tube is the x-ray source. 

There are two possible ways of XRF analysis used in fundamental parameter methods, namely analysis with and without standards. The intensity of the measured characteristic radiation 7 is related to the calculated intensity of radiation /icai... 

Fluorescence Corrections. Fluorescence occurs when the characteristic radiation of a given element (A) is excited by X-ray photons of higher energy than... 

In order to apply fluorescence corrections it is necessary to calculate the intensity of the fluorescence radiation of element A (7f) excited by the characteristic radiation of element B, and to obtain the ratio /f//A, where IA is the intensity of A radiation produced directly by electron bombardment. The fluorescence factor F is given by Ff=l/[l+(/f//A)]. 

For Mo-Ka radiation, 12 = 0.3554A which is produced at a threshold voltage of 34.9 kV. Operating an X-ray source at this potential also results in a drastic reduction in intensity of the desired characteristic radiation [5]. 

The major requirement of the light source for atomic absorption is that it should emit the characteristic radiation (the spectrum) of the element to be determined at a half-width less than that of the absorption line. The natural absorption line width is about 10 4 (A), but due to broadening factors such as Doppler and collisional broadening, the real or total width for most elements at temperatures between 2000 ° and 3000 °K is typically 0.02 — 0.1 A. Hence, a high resolution monochromator is not required. 

Absorption of Characteristic Radiation. Instrumentation. Sample Vaporization. Quantitative Measurements and Interferences. 

Total reflection x-ray fluorescence (TXRF) has become very popular for the conduct of microanalysis and trace elemental analysis [77-79]. TXRF relies on scatter properties near and below the Bragg angle to reduce background interference, and to improve limits of detection that can amount to an order of magnitude or moreover more traditional XRF measurements. As illustrated in Fig. 7.18, if x-rays are directed at a smooth surface at a very small angle, virtually all of the radiation will be reflected at an equally small angle. However, a few x-rays will excite atoms immediately at the surface, and those atoms will emit their characteristic radiation in all directions. One obtains very clean... 

German for braking radiation , as it is emitted when the electrons are braked by the solid. The complete spectrum from a copper target as a function of accelerating voltage is shown in Figure 2.2. It is clear that the characteristic radiation is far more intense than the continuous, and it is used almost exclusively in high resolution diffractometiy. 

In the laboratory. X-ray topography is nsnally performed with characteristic radiation and it is straightforward to show that the exposnre time for a section topograph scales with and for a traverse topograph as where P is the X-ray tnbe power and S is the source dimension in the incidence plane. The continnons... 

In transmission electron microscopy (TEM), a beam of highly focused and highly energetic electrons is directed toward a thin sample (< 200 nm) which might be prepared from solution as thin film (often cast on water) or by cryocutting of a solid sample. The incident electrons interact with the atoms in the sample, producing characteristic radiation. Information is obtained from both deflected and nondeflected transmitted electrons, backscattered and secondary electrons, and emitted photons. 

Unstable radionuclei result on subjecting the nuclei of some elements to neutron bombardment. During the decay process, in which the radionuclei return to more stable forms, characteristic radiation is emitted. The energy of the radiation is characteristic of the element, and its intensity forms the basis for quantitative elemental analysis. The advantages of NAA for trace analysis include low detection limits, good sensitivity, multi-element capability and relative freedom from matrix effects. However, for successful application of this technique skilled personel are required and because of the low sample throughput the amount of work involved in the analysis of column fractions, for example, is prohibitively high. In addition, it may take up to several weeks before the results are available. Further, only few laboratories have easy access to a neutron source. 

Rossi, H.H. (1977). The effects of small doses of ionizing radiation. Fundamental biophysical characteristics, Radiat. Res. 71,1. 

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